[BLDG-SIM] roof anchors and condensation

[Alec Stevens]

Hi,

 

This isn't quite a modeling question, but I figured you all might have some
insight.

 

I am working on a new refrigerated warehouse that is under construction in
New Hampshire.  

The roof assembly is basically a metal corrugated deck, on top of which is
R-50 of rigid foam insulation.  The underside of the deck is exposed to the
refrigerated area.  Roof anchors tie the rigid insulation to the roof deck /
joists.  The rubber membrane roof is adhered to the top of the foam
insulation.  

 

The owner asked whether it made sense to install an additional layer of
rigid insulation above the standard layers.  The new layer would raise the
total assembly to R-63, and would be glued to the top side of the standard
insulation, so that the roof anchors would be embedded in an insulation
sandwich.  We calculated minimal refrigeration savings from the added
R-value.  However, the owner felt that sandwiching the metal anchors in
insulation would remove any threat of condensation and freezing below the
roof membrane where the anchors would potentially be cold enough to cause
this.

 

The owner said that on another building, the roof membrane became separated
at the location of the anchors, and they attributed it to the cold spots,
condensation, and freezing caused by the anchors and their conduit into the
cold space.    Has anyone heard of such a problem?  At first glance, it
would seem that on a hot day, the top end of the metal anchor would be
sufficiently warmed by the hot roof so as not to cause any condensation or
freezing at the membrane surface.  In the winter, the whole roof will be
below freezing, so the extra insulation won't affect things relative to the
anchor then.  Is there another explanation for the failed roofing that the
owner described?  Perhaps the penetration of the anchors through the
insulation is allowing moisture from the space to migrate up to the roof
membrane, and the moisture is trapped and then freezes in the winter?  

 

Any ideas appreciated, as always.  Happy Memorial Day.

 

 

Sincerely,

Alec Stevens, PE, LEED AP

DMI

450 Lexington St., Newton, MA 02466

p: 617-527-1525 x105 f: 617-527-6606

e: astevens at dmiinc.com

 



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